We report the onset of electrochemical doping and subsequent visible light emission at 5V and 360K from a planar light-emitting electrochemical cell with a 1mm interelectrode gap containing poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene] (MEH-PPV), poly(ethylene oxide) (PEO), and XCF3SO3 (X=K,Li) as the active material. We rationalize the unprecedented low turn-on voltage of such wide-gap light-emitting electrochemical cells by demonstrating that the active material contains a mixture of crystalline PEO+XCF3SO3 domains and amorphous MEH-PPV domains at room temperature, but that the crystalline domains have melted at 360K resulting in a significant increase in the ionic conductivity.
We report frozen-junction operation of a polymer light-emitting electrochemical cell containing a mixture of poly͓2-methoxy-5-͑2Ј-ethyl-hexyloxy͒-1,4-phenylenevinylene͔ ͑MEH-PPV͒ and the ionic liquid tetra-n-butylammonium trifluoromethanesulfonate ͑TBA-TF͒ as the active material. We find fast turn-on time, unipolar light emission, and significant operational lifetime up to T = 200 K for planar Au/ ͑TBA-TF+ MEH-PPV͒ / Au surface cells, which had been charged ͑i.e., electrochemically p-and n-type doped in situ͒ at T = 393 K and V = 4 V and then cooled to 80 K at V = 4 V. We employed differential scanning calorimetry to demonstrate that ͑TBA-TFϩMEH-PPV͒ exhibits two melting transitions of TBA-TF crystalline phases located at T m,1 Ϸ 280 K and T m,2 Ϸ 380 K, respectively. The lower T m,1 sets the upper limit for frozen-junction operation ͑with zero-ionic conductivity͒, while the larger T m,2 correlates to the lower limit for the charging regime ͑with high ionic conductivity͒.
SYNOPSISIn this article we present overall crystallization characteristics of five polycaprolactone samples with mean molecular weights ranging from 50,000 to 400,000. The crystallization temperatures and heats of crystallization are determined as a function of mean molecular weight as well as for cooling rates in the range 0.31 to 40 K/min. Our results show a decrease in crystallization temperature from 320 to 300 K a t increasing molecular weight and cooling rate. The heat of crystallization shows a slight decrease within the cooling rate interval and a decrease from about 68 to 48 J/g with increasing molecular weight. We analyze the continuous cooling data according to the Ozawa model for nonisothermal crystallization and compare them with our isothermal data analyzed with the Avrami model. Both the Ozawa and Avrami models give exponent parameters in the range 2.9 to 3.6. In the investigated temperature range and for all samples, we find a nucleation controlled crystallization. At the lowest temperatures, the Ozawa analysis indicates an increasing dependency on limitations in chain mobility. The higher molecular species have in general a slower crystallization rate, with half crystallization times increasing with a factor of about five within the molecular weight range a t 320 K.
Low-temperature heat capacity of fullerite C 60 doped with nitrogen Low Temp.Heat capacities of 1D and 2D pressure polymerized C 60 as well as the thermally depolymerized C 60 have been measured at temperatures between 4 and 350 K and the results analyzed for the lattice vibrations and the thermodynamic stability. It was found from the low-temperature heat capacity (TϽ100 K) that on polymerization ͑1͒ the lattice vibrations, both translational and rotational, stiffen substantially, ͑2͒ an anisotropic nature emerges in response to the lower dimensionality, and ͑3͒ an anharmonicity still remains as in pristine C 60 . The normal C 60 has an excess entropy of 67.8 and 99.4 J K Ϫ1 mol Ϫ1 at 300 K relative to the 1D and 2D polymerized C 60 , respectively. The thermodynamic stability is considered with two possible phase diagrams.
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